The long-term goals of this project are to elucidate the sequence of cellular and molecular events occurring during skeletal muscle development. The project has two components: (a) describing muscle cell lineage from the earliest mesodermal precursor to the multinucleated muscle fiber; and (b) investigating regulatory mechanisms controlling the transit of myoblastic cells from one recognizable compartment to the next. As a means of identifying cells residing in discrete cell lineage states, we have chosen a clonal assay because: (a) it permits us to examine individual cells within a total tissue population which is extremely heterogeneous; and (b) it permits us to obtain viable, clonally restricted, cells whose subsequent differentiation can be manipulated in vitro. The project also involves extensive uses of permanent mouse myoblast cell lines which are used in somatic genetic studies (mouse x mouse and mouse x human) to analyze normal muscle differentiation, human muscle specific gene linkage, and human muscle disease.